1. Field of the Invention
The present invention relates to a radio wave receiver that receives a radio wave and decodes a time code, and a wave clock that performs time calibration based on the time code.
2. Description of Related Art
It is known that a radio wave receiver module receives a standard radio wave including a time code and decodes the time code. It is also known that a wave clock is provided with such a radio wave receiver module for automatically correcting current time based on the decoded time code.
As to a standard radio wave in Japan, for example, amplitude of a carrier wave of 40 kHz or 60 kHz is modulated based on a time code. In areas where radio wave condition is poor, it may be difficult to receive radio waves properly due to radio signal attenuation or exogenous noise contamination.
In view of the foregoing, a technique for achieving reception of radio waves with high sensitivity even under poor radio wave condition is proposed. A technique disclosed in JP-A 2007-139705, for example, prevents a time code from being erroneously detected due to noise components by changing a threshold value for shaping a detected signal, depending on types of standard radio wave.
In a commonly used radio wave receiver module that receives a wide variety of radio waves, a low-pass filter may be provided at an output stage of a detection circuit for removing high-frequency noise in a baseband signal after detection. In such a structure, it is normal that the pass band of the low-pass filter is set so as not to greatly distort a waveform of the baseband signal. For example, assuming that a baseband signal is obtained under the condition that square waves whose pulse widths are 0.2 seconds, 0.5 seconds and 0.8 seconds each having a period of one second, are transmitted, it is normal that a cutoff frequency of the low-pass filter should be set to 5 Hz or higher.
Unfortunately, in the conventional technique for enhancing sensitivity in a radio wave receiver module, the sensitivity is not very high. For example, if a radio wave is contaminated by low exogenous noise, a time code can accurately be decoded. On the other hand, if the noise level is high enough to cause large attenuation of radio signals in a building, the time code cannot accurately be decoded.